1. Field of the Invention
The present invention relates to methods of making a circuit board. More particularly, the present relates to methods of making circuit board, including forming a metal film layer and an insulating layer by the use of an adhesive film with a metal film. The present invention further relates to adhesive films with a metal film, which are preferably used for the production of the aforementioned circuit board.
2. Discussion of the Background
Circuit boards such as multi-layer printed wiring boards, flexible printed wiring boards, and the like, which are widely used for various electronic devices, are desired to have thin layers and permit micro wiring of circuits so as to provide compact electronic devices having high functions. A known example of the production method thereof is a semi-additive method including laminating a curable resin composition using an adhesive film on an inner layer circuit board, curing the curable resin composition to form an insulating layer, roughening the insulating layer with an oxidant such as alkaline potassium permanganate solution and the like, forming a plating seed layer on the crude surface by electroless plating, and forming a conductive layer by electroplating. This method requires an anchor effect between the conductive layer and the insulating layer by roughening the surface of the insulating layer with an oxidant (forming concaves and convexes on the surface) as mentioned above, so as to confer high adhesion strength to the conductive layer. However, the method is associated with problems in that the seed layer of the anchor part resists removal of unnecessary plating seed layer by etching during formation of circuits, and etching under the conditions capable of sufficiently removing the seed layer of the anchor part noticeably dissolves the wiring patterns, thus preventing micro wiring.
To solve these problems, transfer of a metal film layer to be a plating seed layer onto an adherend (transferred body) using a transfer film has been tried. For example, JP-A-2004-230729 and JP-A-2002-324969 disclose a method for preparing a transfer film wherein a metal film layer is formed on a support via a release layer by vapor deposition and the like, transferring the metal film layer of the transfer film onto the surface of a resin composition layer (insulating layer) on the substrate or the surface of a prepreg, and forming a conductive layer by plating and the like on the transcribed metal film layer. The method of JP-A-2004-230729 is a method using a transfer film including, as a release layer, fluororesin, polyolefin resin or polyvinyl alcohol resin, and the method of JP-A-2002-324969 is a method using a transfer film including, as a release layer, an adhesive containing an adhesive resin such as acrylic resin, melamine resin. On the other hand, JP-A-9-296156 discloses an adhesive film obtained by directly forming a metal film layer on a support by vapor deposition and the like, and forming a resin composition layer thereon.